Avoid a crash in source_cache::extract_lines

If the first requested line is larger than the number of lines in the
source buffer, source_cache::extract_lines could crash, because it
would try to pass string::npos" to string::substr.

This patch avoids the crash by checking for this case.

This version of the patch changes get_source_lines to return
std::string.

gdb/ChangeLog
2019-03-14  Tom Tromey  <tromey@adacore.com>

	* source-cache.h (class source_cache) <get_source_lines>: Return
	std::string.
	* source-cache.c (source_cache::extract_lines): Handle case where
	first_pos==npos.  Return std::string.
	(source_cache::get_source_lines): Update.

1 files changed, 4 insertions, 3 deletions

diff --git a/gdb/source-cache.h b/gdb/source-cache.h
index dd23266..e2e25a1 100644
--- a/gdb/source-cache.h
+++ b/gdb/source-cache.h
@@ -65,9 +65,10 @@ private:
 			       int last_line, std::string *lines_out);
   /* A helper function for get_plain_source_lines that extracts the
      desired source lines from TEXT, putting them into LINES_OUT.  The
-     arguments and return value are as for get_source_lines.  */
-  bool extract_lines (const struct source_text &text, int first_line,
-		      int last_line, std::string *lines_out);
+     arguments are as for get_source_lines.  The return value is the
+     desired lines.  */
+  std::string extract_lines (const struct source_text &text, int first_line,
+			     int last_line);
 
   /* The contents of the cache.  */
   std::vector<source_text> m_source_map;